Periodontal disease causes significant morbidity at considerable costs to the federal government health care systems. If the clinician can identify such patients earlier for instituting preventive measures, this financial burden can be reduced. It is helpful to identify the presence of specific microorganism which can be used as an indicator of different stages or types of periodontal diseases. One such organism is the oral protozoa Entamoeba gingivalis whose presence has been reported but its role in the pathogenesis of periodontal disease has not been clarified. Recent developments of xenic culture methods have made possible the growth of Entamoeba from subgingival plaque samples of dental patients. On the basis of electrophoretic patterns of enzymes, strains of Entamoeba histolytic have been classified as pathogenic (occurring in hepatic amebiasis patients) and nonpathogenic strains (occurring in asymptomatic carriers). It is proposed to determine if this technique can also be applied to classify E. gingivalis strains as pathogenic (found in various periodontal diseases) and nonpathogenic. The following are the specific aims for this study: (a) Presence of E. gingivalis will be identified by phase contrast microscopy of undisturbed subgingival plaque samples from dental patients of Veterans Administration Medical. Center, North Chicago, IL. (b) The amoeba will be grown in xenic medium and the soluble extracts will be analyzed for isoenzyme patterns by agarose gel electrophoresis. (c) Based on the effect of amoeba or the extracts, on the destruction of monolayers of human gingival fibroblasts (direct pathogenicity) or on the destruction of polymorphonuclear neutrophils (indirect pathogenicity), strains will be classified as pathogenic or nonpathogenic strains.